Radiation is converted to electron hole pairs in semiconductor material. In semiconductor radiation detectors the electron hole pairs are separated by an electric field. The measured charge type is referred to as signal charge and the opposite charge type is referred to as secondary charge.
The patent applications WO 2006/018470 A1, WO 2006/018477 A1, PCT/FI2006/000009 and PCT/FI2006/000058 which are included herein by reference disclose a semiconductor radiation detector having a modified internal gate (MIG). The MIG detector presented in the patent applications WO 2006/018470 A1, WO 2006/018477 A1 is optimized for the detection of low energy X-rays and for particle and near infra-red radiation in case the semiconductor material is silicon. The MIG detector presented in the last two applications is optimized for the detection of visible light in low light level circumstances in case the semiconductor material is silicon. The MIG detectors are comprised of a bulk layer, of a MIG layer of the second conductivity type on top of the bulk layer, of a barrier layer of the first conductivity type on top of the MIG layer and of pixel dopings of the second conductivity type on top of the barrier layer. There may be also channel stop dopings of the first conductivity type on top of the barrier layer. The MIG detectors enable non-destructive reading of the signal charge, isolation between the signal charges and the surface generated charges and it has a low capacitance. For these reasons the MIG structure provides the best possible detection sensitivity of all semiconductor radiation detectors.
The problem associated with the MIG detectors is, however, a low dynamic range. This is due to the low full well capacity of the MIG. The row reset or the rolling shutter mechanism provides an equal integration time for each pixel which improves the image quality especially if a short integration time is used. The MIG detectors of PCT/FI2006/000009 and PCT/FI2006/000058 comprise an additional clear contact (e.g. 1334 in PCT/FI2006/000058) of the second conductivity type, which can be used also as an anti blooming drain, and a clear gate (e.g. 1343 in PCT/FI2006/000058) controlling the flow of signal charges from the MIG layer to the clear contact. This arrangement enables row reset since the clear gates of a row of pixels can be interconnected and a reset signal can be provided to the interconnected row of gates. The afore mentioned arrangement enables actually individual reset of pixels since the pixel doping (e.g. 1333 in PCT/FI2006/000058) closest to the clear gate can function also as an additional clear gate beside being a drain. This requires that these pixel dopings are connected for instance row wise in the pixel matrix and that the clear gates are connected column wise in the pixel matrix. The problem with the individual reset arrangement is that a large current may run between the drain and the clear contact during the reset operation which enhances the power consumption of the device.
A problem of the row reset related especially to still images is that images of fast moving objects are blurred since the start and the end points of the integration period are different in different rows although the integration time is the same.